That's the question NASA seeks to answer with the scheduled launch Monday of the MAVEN spacecraft.

Planetary scientists believe the answer lies high in the Martian atmosphere. Today, it's a thin layer of mostly carbon dioxide gas. But long ago, it may have been thick enough to host a life-friendly, even Earth-like, environment. If so, the Mars Atmosphere and Volatile Evolution mission may reveal clues about where it all went.

Previous missions, from rovers to orbiters, have tried to see into Mars' past by examining the Red Planet's surface. But that only tells part of the story, said Bruce Jakosky, a planetary scientist at the University of Colorado Boulder and principal investigator for the MAVEN mission.

"If we want to understand Mars as a system, we need to include the role of the upper atmosphere," he said. "We can no longer just look at the geology and understand it in isolation."

Earth is flanked by two alternative worlds. On the side closer to the Sun is Venus, a planet with too much atmosphere, boiling away under thick layers of carbon dioxide. On the other side lies Mars, a planet with too little air shielding its cold, dead deserts.

"We think all three of those planets, when they formed, were not all that different," said Steven Lee, curator of planetary science at the Denver Museum of Nature and Science. Life arose on Earth because it's a "Goldilocks" case, protected by just the right amount of air.

Robotic explorers sent to Mars by NASA and other space agencies have already picked up many signs that water once flowed across the surface.

Scientists have identified craters that they think were once filled with lakes, along with clay minerals that must have been formed by exposure to water.

"I wouldn't be surprised if, for many tens of millions of years, Mars was a pretty friendly place with natural water, wind, waves and rain," said Paul Mahaffy, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. who leads instruments on both MAVEN and the Curiosity rover. "But I think we just don't know."